Diversity receivers exploit transmit redundancy to gain reception performance improvements. For example, so-called multi-branch receivers generally include two or more receive antennas, with each antenna providing an antenna-specific version of the same received signal. Assuming some minimal spatial separation for the antennas, the received signal exhibits generally uncorrelated fading behavior across the antennas, and each antenna thus provides a different diversity signal for processing.
Multi-antenna designs, however, bring with them inherent cost disadvantages. Further, as a practical matter, fitting even the first decently performing antenna into a small portable communication device is challenge enough. Few designers welcome the added burden of finding room for additional, spatially separated diversity antennas.
However, a receiver can operate with a form of diversity reception even without the presence of multiple receiver antennas. For example, the in-phase (I) and quadrature (Q) components of a received signal can be treated as diversity signals in a Spatial-Temporal-Whitening (STW) process. Indeed, STW processing represents a core aspect of the Single-Antenna-Interference-Cancellation techniques of particular interest in certain types of wireless communication networks, such as GSM and EDGE networks.
Such networks use of form of Time-Division-Multiple-Access (TDMA) transmission wherein multiple users in the same cell or sector share the same carrier frequency, but are assigned different times—slots—for transmitting and receiving data. However, frequency reuse within the network means that nearby sectors use the same frequencies and time slots to transmit different data to other users, giving rise to significant levels of co-channel interference, which, along with adjacent-channel interference caused by spectral splatter and other phenomena, constrains system capacity and/or compromises service quality.
Better interference cancellation in these and other types of communication networks directly improves network capacity and service quality. Thus, considerable interest surrounds the topic of improved receiver performance. For example, the Downlink Advanced Receiver Performance (DARP) standard significantly tightens the performance requirements for interference cancellation for receivers of the type operating in GSM/EDGE systems.
Over-sampling represents one mechanism for further improving SAIC processing, as a means of meeting enhanced interference cancellation performance required by the DARP standard. Sampling the received signal at a multiple of the minimum sampling rate “artificially” creates diversity signals, i.e., diversity sample sets, wherein each diversity signal represents a different sampling phase of the over-sampled signal.
Whether artificially created through over-sampling, or whether derived from multiple receiver antennas, the use of diversity signals can improve receiver performance. However, one typical assumption, particularly in the case of over-sampling receivers, is that the diversity signals naturally are symbol aligned with one another. However, delay variations in the propagation channel, sampling phase mismatches in anti-aliasing filters, analog-to-digital converter and decimation filter mismatches, etc., can all contribute to symbol misalignment between the diversity signals. Thus, the assumption of natural symbol alignment between the diversity signals may be incorrect, meaning that any signal combining or other diversity-processing operations performed on the diversity signal will yield less-than-optimum results.